iptables is like alchemy

[Alan Evans <ame.fedora@xxxxxxxxx>]

This is really related to iptables, not I presume Fedora-specific. But I'm really hoping that somebody here will be able to school me on iptables, so I don't have to find and subscribe to some other list just to ask one question.

I'm faced with the problem of needing to punch a hole in a firewall on our portal server so that, in our case, ssh to port 20022 on external interface of that server actually just connects to port 22 on another machine located in the network on the internal interface. I hope I'm being clear.

I've tried many iterations of iptables rules to accomplish this. The closest I've come is:

iptables -A PREROUTING -t nat -p tcp -s 0/0 --dport 20022 -j DNAT --to 192.168.0.35:22

And indeed connecting to port 20022 on portal just goes straight to port 22 on the other (192.168.0.35) machine. The problem is, as soon as I apply this rule, DNS queries (portal is also a DNS server) to the external interface stop working.

I've googled endlessly and found about a thousand variations by people that are each supposed to solve a subtly different variation on what I'm trying to do. Nothing I've tried does what I want without bad side effects like I describe above.

-Alan

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